Re: [Xen-users] Re: Partition vs disk images

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> > In RHEL 5 at least, anaconda requires the block device/file to be mapped
> > as disk (e.g hda, sda) not partition (e.g. sda1). Try changing it to
> > disk = [ "phy:lvm-raid/FileVolGroup,sda,w" ]
>
> Ok, so (at least in CentOS/RHEL?) the Xen domU sees the dom0 partition not
> as a partition but as a physical disk?

That depends on whether you put sda (whole disk) or sda1 (partition) in the 
config quoted above.  That's a standard Xen thing, not specific to 
CentOS/RHEL.

What I believe Fajar was saying is that the *installer* used by CentOS/RHEL 
guests will be unhappy if you try and provide it with separate partitions 
instead of a whole disk, which limits what you can do.

> sda1, sda, and hda all failed 
> (sda1, sda were not visible while hda kept generating installer errors
> detecting the drives).  hda1 shows up as weirdly named disk  (/dev/hda1)
> that I need to partition out in the domU installer (ie, /dev/hda11,
> /dev/hda12, etc).

Wow.  That's ... different to normal!  I think we can safely say your 
installer didn't like that, then!

> disk = [ "phy:lvm-raid/FileVolGroup,hda1,w",
>          "phy:lvm-raid/io-swap,hda2,w" ]     # assumption was that these
> were partitions...

They usually are, but it seems your guest's installer is expecting only a 
whole virtual block device exported to it, not separate partitions.  I think 
that's probably resulting in the weird behaviour described above.

Another thing: assuming you're installing a PV guest, try using xvda instead 
of hda.  I'm not sure if this will solve your problem with the separate 
partitions, but if you end up falling back to using a whole device it should 
avoid the installer complaining so much!

> What concerns me is that if/when I want to grow disk available to a domU
> (this on in particular is a file server), I can't just grow what shows up
> as hda1, right?  I'd need to add a new "disk" and extend a LVM within the
> domU?

Alternatively you could extend the whole drive then resize the partitions 
within it.

> Don't these layers of RAID+LVM (dom0) and Xen block device and LVM (domU)
> come at a price?  I was hoping that the partition got mapped straight into
> the domU so I avoided any extra stuff within the domU and I would have the
> added bonus of being able to mount the drives and copy files between when
> setting this up.

Indeed.  Xen supports this, your guest OS installer may not.  It's probably 
possible to "persuade" your guest OS to run on per-partition virtual devices 
if you really want that but it would require a bit of extra fiddling.

I believe you can access partitions within an arbitary block device using the 
kpartx tool, which may be useful for poking into a guest's virtual disk.

> > Having said that, if you're building lots of identical domUs, using
> > prebuilt template (and mappingthe block device  as sda1 instead of sda)
> > should be faster than installer.
>
> If I can only do partitions, how does this work then?  Unless all the
> drives are identical in size and I 'dd' the device??

I think you'd want to use a pre-installed template guest OS (possibly having 
made modifications to make it run as you want) and then you'd copy it to 
create new ones instead of doing an install+customise.  You'd dd to duplicate 
contents to other block devices in dom0, or cp to copy a file-based VBD.

Cheers,
Mark

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Push Me Pull You - Distributed SCM tool (http://www.cl.cam.ac.uk/~maw48/pmpu/)

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